Get a merc maint manual and go through the fuel pressure test scenario and get msd heli-coil solid conductor plug wires if you unsure what wires are on your motor. As for retarding timing, what do you mean

Well it's fixed.

I'm not sure which one fixed it but here is what we changed, fixed, or set.

1) Added fuel boost pumps to the fuel system.
2) Changed cap and rotor again
3) Changed plug wires (had the wrong wires on it.)
4) Changed plugs both engines
5) Both engines timing was too high (reset base timing)
6) Changed knock sensor on Starboard engine.
7) Changed IAC motor on Starboard engine.
8) Had props refurbished and tuned (Thanks Throttle-Up)

The timing was really bad on the port engine. You could tell when we started moving the distributor it started to sound better. Did a couple high rpm runs this evening, and no problems like before. Mercury put the boost pumps on the later 500's for a reason, so we figured it would not hurt to add them to our engines. We should have good fuel pressure no matter what rpm we are running now.

Now it's time to enjoy it.

My money goes on the "TIMING".